Background The Deepwater Horizon disaster cleanup effort provided an opportunity to examine the effects of ambient thermal conditions on exertional heat illness (EHI) and acute injury (AI). Methods The outcomes were daily person-based frequencies of EHI and AI. Exposures were maximum estimated WBGT (WBGTmax) and severity. Previous day's cumulative effect was assessed by introducing previous day's WBGTmax into the model. Results EHI and AI were higher in workers exposed above a WBGTmax of 20 degrees C (RR 1.40 and RR 1.06/degrees C, respectively). Exposures above 28 degrees C-WBGTmax on the day of the EHI and/or the day before were associated with higher risk of EHI due to an interaction between previous day's environmental conditions and the current day (RRs from 1.0-10.4). Conclusions The risk for EHI and AI were higher with increasing WBGTmax. There was evidence of a cumulative effect from the prior day's WBGTmax for EHI. (C) 2016 Wiley Periodicals, Inc.